The present invention is directed to analog-to-digital converter apparatuses, and especially to current mode pipeline analog-to-digital converter apparatuses that include residue amplifiers in a plurality of stages.
A current mode pipeline analog-to-digital converter (ADC) usually employs a plurality of stages, each stage including a residue amplifier operating in cooperation with a track-and-hold device. In its preferred embodiment, the residue amplifier receives a differential analog signal at two amplifier units. A DC (direct current) level setting current signal is also provided to the two amplifier units to set the DC levels, and ultimately to set the common mode voltage for the two amplifier units. A preferred source for such DC level setting current signals is an NPN digital-to-analog converter (DAC). When an NPN DAC is used to provide DC level setting currents in a current mode pipeline ADC, it has been found useful to employ a PNP DAC to provide a control current signal to the two amplifier units to recentralize the common mode voltage of the output of the residue amplifier.
In configurations employing both a PNP DAC and an NPN DAC, both DAC devices are first order noise sources contributing noise that can be manifested in the output of a residue amplifier.
Noise is always a concern for designers of electronic devices, including ADC devices. It is desirable that noise in the output of the residue amplifier devices in an ADC apparatus be minimized in so far as can be achieved while retaining other advantageous operational characteristics.